1. Field of the Invention
The present invention relates to an optical fiber for a long-haul optical transmission system.
2. Description of the Related Art
With an increase of an amount of communication information, a wavelength-division-multiplexing (WDM) transmission is widely used in a field of communication, using an optical signal in a wavelength band centering around 1550 nanometers where a lowest transmission loss can be expected in a silica-glass-based optical fiber. The WDM transmission is a system transmitting a plurality of optical signals of different wavelengths through a single optical fiber.
In the WDM transmission, major factors restricting a long-distance transmission of the optical signal with a high speed include a transmission loss of the optical fiber, a wavelength dispersion, and a nonlinearity. If the transmission loss is high, degradation of the optical signal becomes large, which restricts a transmission distance. Factors that cause the transmission loss include a Rayleigh scattering, a scattering due to a structural defect in a core, and optical absorption due to impurities, such as a hydroxyl (OH) group, in the core.
A conventional optical fiber has a refractive index profile designed to confine an optical signal in the core for propagating the optical signal by doping germanium (Ge), which is a dopant for increasing a refractive index, into the core to provide a refractive index difference between the core and a cladding formed with a silica glass without a dopant (pure silica glass). On the other hand, as an optical fiber for realizing a low transmission loss, an F-doped-cladding fiber is known in which a refractive index profile is employed which provides a refractive index difference between the core and the cladding by decreasing the refractive index of the cladding by doping fluorine (F), which is a dopant for decreasing the refractive index, into the cladding while maintaining the refractive index of the core as the refractive index of the pure silica glass without doping virtually any dopant into the core. Because there is virtually no dopant doped in the core, the F-doped-cladding fiber can lower the transmission loss caused by the dopant, which is an impurity, compared to the optical fiber having a Ge-doped core.
However, viscosity of the silica glass becomes low if a dopant such as F and Ge is doped. As a result, in the case of the F-doped-cladding fiber, a stress at the time of drawing is concentrated on the core that includes virtually no dopant, so that the stress is remained in the core, resulting in an occurrence of a structural defect. The structural defect that has occurred in such a manner becomes a cause of the transmission loss. To reduce the residual stress in the core, a method is disclosed in U.S. Pat. No. 6,917,740, in which a low transmission loss is obtained by reducing a glass transition temperature difference by adjusting a concentration of a dopant such as F and chlorine (Cl) in the core and an F-doped cladding layer that is adjacent to the core to match the viscosities of the core and the F-doped cladding layer. In U.S. Pat. No. 6,917,740, a transmission loss of 0.18 dB/km is disclosed.
When the wavelength dispersion is larger, a waveform distortion of a propagating optical signal is increased with a consequent result that a high-speed transmission cannot be achieved. On the other hand, if the wavelength dispersion reaches zero, a four-wave mixing (FWM), which is one of the nonlinear optical phenomena, occurs, so that the WDM transmission becomes difficult.
To cope with the above problems, as a means for suppressing a negative effect due to both the wavelength dispersion and the nonlinear optical phenomena, a dispersion-compensating optical transmission path is disclosed in Japanese Patent Application Laid-Open No. 2001-91782, in which the wavelength dispersion becomes close to zero on an entire optical transmission path by forming the optical transmission path by connecting two types of optical fibers including an optical fiber having a positive dispersion in a transmission band of the optical signal and an optical fiber having a negative dispersion. In the dispersion-compensating optical transmission path, an occurrence of the nonlinear optical phenomena is suppressed by employing an optical fiber having a large effective core area, for example, 75 μm2, at a stage preceding an optical transmission path to which a high-power optical signal is input.
However, because a high transmission loss not only restrict the transmission distance but also necessitates to increase output power of an optical amplifier for compensating the transmission loss, there are such problems that a cost of the entire optical transmission system is increased and the nonlinear phenomena occurs with ease. These problems become increasingly prominent with an increase of the number of optical signals and an increase of the transmission distance in the WDM transmission. Therefore, for achieving an even higher performance of the optical transmission system, an optical fiber having a lower transmission loss is highly demanded.